1. Field of the Invention
The present invention relates to a heating system, not exclusively but preferably, used as a heating system for heating an objective heated area of a vehicle such as a passenger compartment. More particularly, the present invention relates to a vehicle heating system accommodating therein a viscous fluid type heat generator which employs viscous fluid to generate heat by the application of a shearing force thereto and transmits the heat to a circulating heat exchanging fluid, typically an engine coolant (cooling water), capable of carrying the heat to the objective heated area in the vehicle. The present invention also relates to a method of controlling the vehicle heating system.
2. Description of the Related Art
Japanese Unexamined Patent Publication (Kokai) No. 2-246823 (JP-A-2-246823) discloses a typical vehicle heating system in which a viscous fluid type heat generator, able to generate heat by using a viscous fluid frictionally generating heat when it is subjected to a shearing action, is incorporated.
The viscous fluid type heat generator disclosed in JP-A-2-246823 includes a pair of mutually opposing front and rear housings tightly secured together by appropriate tightening elements, such as through bolts, to define an inner heat generating chamber and a heat receiving chamber arranged around the heat generating chamber in the form of a water jacket. The heat-generating chamber is formed as a fluid-tight chamber and is isolated from the heat-receiving chamber by a partition wall through which the heat is exchanged between the viscous fluid in the fluid-tight heat-generating chamber and the engine coolant (the heat exchanging fluid) in the heat-receiving chamber. The coolant is introduced into the heat receiving chamber through a water inlet port and delivered from the heat-receiving chamber toward an external heating system, and the water is constantly circulated through the heat generator and the external heating system.
A drive shaft is rotatably supported in the front housing via an anti-friction bearing so as to support thereon a rotor element in such a manner that the rotor element is rotated with the drive shaft within the fluid-tight heat-generating chamber. The rotor element has outer faces which are in face-to-face with the inner wall surfaces of the fluid-tight heat generating chamber and form therebetween a small gap in the shape of labyrinth grooves, and a viscous fluid, e.g., silicone oil, is supplied into the heat generating chamber so as to fill the small gap, i.e., the labyrinth grooves between the rotor element and the wall surfaces of the fluid-tight heat generating chamber.
When the drive shaft of the viscous fluid type heat generator incorporated in the vehicle heating system is driven by an engine of a vehicle via a solenoid clutch, the rotor element is rotated within the heat generating chamber so as to apply a shearing action to the viscous fluid held between the wall surfaces of the fluid-tight heat generating chamber and the outer faces of the rotor element. Thus, the viscous fluid (silicone oil) generates heat due to the shearing action applied thereto. The heat is transmitted from the viscous fluid to the coolant flowing through the heat-receiving chamber. The coolant carries the heat to the heating circuit of the vehicle heating system to heat an objective heated area, e.g., a passenger compartment of the vehicle.
In the described viscous fluid type heat generator, connection and disconnection of the solenoid clutch are conducted on the basis of a control signal indicating only the temperature of the coolant which must be always circulated through a water jacket of the vehicle engine for the purpose of cooling the vehicle engine. Therefore, when the temperature of the coolant is lower than a preset temperature value, the solenoid clutch is connected to drive the rotor element of the viscous fluid type heat generator. As a result, even if the temperature of the viscous fluid within the heat-generating chamber is excessively high, the viscous fluid is continuously subjected to the shearing action applied by the rotating rotor element. Thus, the viscous fluid, e.g., the silicone oil is thermally and mechanically degraded or deteriorated to reduce its heat-generating performance. It should be understood that an upper permissible temperature of the silicone oil is considered to be approximately 200.degree. C., and if the temperature of the silicone oil exceeds the upper permissible temperature, the thermal degradation of the silicone oil and the mechanical degradation thereof due to an application of a shearing action easily occur.
Alternately, if connection and disconnection of the solenoid clutch between the vehicle engine and the drive shaft of the heat generator is conducted on the basis of a control signal indicating only a rotating speed of the vehicle engine per unit time (the rotating speed of the vehicle engine) and in turn a rotating speed of the rotor element per unit time (the rotating speed of the rotor element), it may be possible to eliminate the above-mentioned defect of the viscous fluid type heat generator of JP-A-2-246823. Then, as shown in FIG. 6, even when the temperature of the coolant is either at -40.degree. C. or at 80.degree. C., the solenoid clutch will be disconnected when the rotating speed of the vehicle engine, i.e., that of the rotor element is increased to a preset number. Thus, the shearing force is applied to the viscous fluid within the heat-generating chamber in direct connection with the rotating speed of the vehicle engine, and in turn that of the rotor element. Accordingly, even when the vehicle is continuously operated and runs at a given speed, the rotor element of the viscous fluid type heat generator driven by the vehicle engine, via the solenoid clutch, will be automatically disconnected from the vehicle engine as soon as the rotations of the rotor element exceeds the preset number to prevent the application of the shearing action to the viscous fluid by the rotor element, and the degradation of the viscous fluid can be avoided.
However, when the connection and disconnection of the solenoid clutch is conducted on the basis of the detection of the rotating speed of the vehicle engine and that of the rotor element, it occurs that the rotation of the rotor element is completely stopped due to the disconnection of the solenoid clutch, and heat generation by the viscous fluid is resultingly stopped even if an objective heated area is cold. Therefore, it becomes impossible to adjustably control the heat generating performance of the viscous fluid type heat generator. Thus, for example, when a vehicle is operated at such a given high speed that the rotating speed of the vehicle engine is far above the preset rotating speed of the rotor element of the viscous fluid type heat generator before the coolant has been sufficiently heated by the viscous fluid type heat generator, the solenoid clutch is left disconnected to thereby prevent transmission of the drive force from the vehicle engine to the rotor element of the viscous fluid type heat generator and, accordingly, the viscous fluid type heat generator cannot generate heat to be used for heating an objective heated area, e.g., a passenger compartment of the vehicle even if the heated area is cold.